1. Field of the Invention
The present invention relates to a method of compensating the level of an output signal from an oxygen concentration sensor of an internal combustion engine.
2. Description of Related Art
In order to reduce exhaust gas pollutants and to improve the fuel consumption of an internal combustion engine, it is now common practice to employ an oxygen concentration sensor to detect the concentration of oxygen in the engine exhaust gas, and to execute feedback control of the air-fuel ratio of the mixture supplied to the engine such as to maintain the air/fuel ratio at a target value. This feedback control is performed in accordance with an output signal from the oxygen concentration sensor.
One form of oxygen concentration sensor which can be employed for such air/fuel ratio control functions by producing an output signal which varies in level in proportion to the oxygen concentration in the engine exhaust gas. Such an oxygen concentration sensor has been disclosed for example in Japanese patent laid-open No. 52-72286. This sensor consists of an oxygen ion-conductive solid electrolytic member formed as a flat plate having electrodes formed on two main faces, with one of these electrode faces forming part of a gas sampling chamber. The gas sampling chamber communicates with a gas which is to be measured, i.e. exhaust gas, through a lead-in aperture. With such an oxygen concentration sensor, the oxygen ion-conductive solid electrolytic member and its electrodes function as an oxygen pump element. By passing a flow of current between the electrodes such that the electrode within the gas sampling chamber becomes a negative electrode, oxygen gas within the gas sampling chamber adjacent to this negative electrode becomes ionized, and flows through the solid electrolytic member towards the positive electrode, to be thereby emitted from that face of the sensor element as gaseous oxygen. The current which flows between the electrodes is a boundary current value which is substantially constant, i.e. is substantially unaffected by variations in the applied voltage, and is proportional to the oxygen concentration within the gas under measurement. Thus, by sensing the level of this boundary current, it is possible to measure the oxygen concentration within the gas which is under measurement. However if such an oxygen concentration sensing apparatus is used to control the air/fuel ratio of the mixture supplied to an internal combustion engine, by measuring the oxygen concentration within the engine exhaust gas, it will only be possible to control the air/fuel ratio to a value which is in the lean region, relative to the stoichiometric air/fuel ratio. It is not possible to perform air/fuel ratio control to maintain a target air/fuel ratio which is set in the rich region.
An oxygen concentration sensor which will provide an output signal level varying in proportion to the oxygen concentration in engine exhaust gas for both the lean region and the rich region of the air/fuel ratio has been proposed in Japanese patent laid-open No. 59-192955. This sensor consists of two oxygen ion-conductive solid electrolytic members each formed as a flat plate, and each provided with electrodes. Two opposing electrode faces, i.e. one face of each of the solid electrolytic members, form part of a gas holding chamber which communicates with and retains a gas under measurement, via a lead-in aperture. The other electrode of one of the solid electrolytic members faces into the atmosphere. In this oxygen concentration sensor, one of the solid electrolytic members and its electrodes functions as an oxygen concentration ratio sensor cell element. The other solid electrolytic member and its electrodes functions as an oxygen pump element. If the voltage which is generated between the electrodes of the oxygen concentration ratio sensor cell element is higher than a reference voltage value, then current is supplied between the electrodes of the oxygen pump element such that oxygen ions flow through the oxygen pump element towards the electrode of that element which is within the gas sampling chamber. If the voltage developed between the electrodes of the sensor cell element is lower than the reference voltage value, then a current is supplied between the electrodes of the oxygen pump element such that oxygen ions flow through that element towards the oxygen pump element electrode which is on the opposite side to the gas holding chamber. In this way, a value of current is obtained which varies in proportion to the oxygen concentration of the gas under measurement, both in the rich and the lean regions of the air/fuel ratio.
However if air/fuel ratio control is executed by using such an oxygen concentration sensor, producing an output varying in proportion to oxygen concentration, then if air/fuel ratio control is initiated after completion of activation of the oxygen concentration sensor during a period of warming-up operation of the engine, the gas which is passed to the sensor will have been produced with incomplete combustion taking place in the engine, so that the output from the sensor will contain a component which results from this incomplete combustion. FIG. 1 is a graph for comparing operation during the warm-up period and operation after warm-up has been completed. As shown, during the warm-up period, the level of pump current which is the output value from the oxygen concentration sensor (and which represents the air/fuel ratio of the mixture supplied to the engine) is higher than the value which is obtained during operation after warm-up has been completed. As a result, problems arise since it is not possible to accurately judge the air/fuel ratio of the mixture which is being supplied to the engine on the basis of the output signal level from the oxygen concentration sensor.